Apparatus and method for modifying an edge

ABSTRACT

Edge modifying apparatuses that include a disc chuck, a disc edge modifying unit having an abrasive article support, and a uniform velocity linear oscillation driver coupled to at least one of the disc chuck and the abrasive article support are described. The uniform velocity linear oscillation driver provides a substantially constant velocity for at least  80 % of each half-cycle of oscillation. Methods of modifying the edge of disc using such apparatuses are also described.

FIELD

This disclosure pertains to edge modifying apparatuses. Specifically, the apparatuses include a mechanism for providing linear oscillation of an abrasive article relative to the edge of a work piece at a substantially constant velocity. Methods of modifying the edge of a work piece are also disclosed.

BACKGROUND

Rigid cylindrical discs are common intermediates in a variety of commercial enterprises. For example, in the manufacture of semiconductor devices, silicon wafers are obtained by slicing a solid cylindrical ingot. Other useful materials include glass, glass ceramics, ceramics, gallium arsenide, sapphire, and compound semiconductor substrates.

Generally, the peripheral edges of the cylindrical discs may have defects including small sub-surface cracks or chips. These defects can create problems in subsequent processing steps, e.g., some defects can propagate from the peripheral edge to the interior of the disc. In addition, as initially produced, cylindrical discs may have sharp edges that are prone to chipping and can be hazardous to handle. Also, after one or more processing steps, undesirable debris may be present at or near the edge of the disc.

Various types of grinding machines have been used to modify such peripheral edges both to minimize or eliminate defects and to round, bevel, or otherwise contour sharp edges. Grinding has been conducted using both grinding wheels comprising abrasive particles fixed in a metal or resin matrix, and fixed abrasive webs comprising abrasive particles fixed in a matrix and adhered to a flexible backing. In addition, polishing slurries have been used wherein the edge of the disc contacts a polishing pad in the presence of a slurry of abrasive particles in a liquid medium.

SUMMARY

In one aspect, the present disclosure provides an edge modifying apparatus comprising a disc chuck including a disc-mounting surface and an axis of rotation; a disc edge modifying unit comprising an abrasive article support having an axis of rotation and a support surface; and a uniform velocity linear oscillation driver coupled to at least one of the disc chuck and the abrasive article support. In some embodiments, the uniform velocity linear oscillation driver is mechanically coupled to the abrasive article support. In some embodiments, the uniform velocity linear oscillation driver comprises a cam having a peripheral edge defined by a curve resulting in a substantially constant linear oscillation velocity during at least 80% of each half-cycle of oscillation. In some embodiments, the uniform velocity linear oscillation driver further comprises a cam follower mechanically coupled to the peripheral edge of the cam, wherein the peripheral edge is defined by a curve resulting in a substantially constant oscillation velocity during at least 90% of each half-cycle of oscillation. In some embodiments, the disc chuck axis of rotation forms an angle of between about 30 degrees and about 75 degrees, inclusive, relative to the support axis of rotation.

In another aspect, the present disclosure provides a method of modifying the edge of a disc. In some embodiments, the method comprises providing the edge modifying apparatus; centering a disc on the disc-mounting surface of the disc chuck; positioning an edge of the disc relative to a fixed abrasive article adjacent the support surface of the abrasive article support such that at least a portion of the edge of the disc contacts an abrasive surface of the fixed abrasive article; rotating the disc chuck; and providing an linear oscillation of the fixed abrasive support surface relative to the disc-mounting surface, wherein the linear oscillation has a substantially constant velocity for at least 80% of each half-cycle of oscillation.

The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary edge modifying apparatus according to one embodiment of the present disclosure.

FIG. 2 illustrates a cam according to one embodiment of the present disclosure.

FIG. 3 illustrates the displacement provided by the cam of FIG. 2.

DETAILED DESCRIPTION

Referring to FIG. 1, an exemplary edge modifying apparatus according to one embodiment of the present disclosure is shown. Edge modifying apparatus 100 includes disc holding unit 200, and disc edge modifying unit 300.

Generally disc-holding unit 200 includes disc chuck 210 having disc-mounting surface 215. In a typical operation, a disc is releasably held against the disc-mounting surface. In some embodiments, the disc chuck is a vacuum chuck. Generally, a vacuum chuck includes a vacuum source to create a vacuum at the disc-mounting surface sufficient to hold the disc in place during the edge modifying operation. In some embodiments, adhesives, e.g., pressure sensitive adhesives and non-pressure sensitive adhesives, may be used to hold the disc against the disc-mounting surface. Other known means of holding the disc against the disc-mounting surface, e.g., mechanical devices, may also be used.

In some embodiments, disc-holding unit 200 includes positioning assembly 220. The positioning assembly is used to bring edge 255 of disc 250 in contact with, e.g., abrasive article 320. In some embodiments, the positioning assembly can be used to apply a contact force, e.g., a fixed contact force, between the edge of the disc and the surface of the abrasive article. In some embodiments, the positioning assembly can be used to retract the disc from the abrasive article during and/or following the edge modifying operation.

In some embodiments, the positioning assembly comprises a piston connected to a pneumatic or hydraulic source. In some embodiments, the positioning assembly comprises a motor mechanically coupled to the disc holding unit. Other mechanisms for the controlled positioning of one element relative to another are well known in the art.

Disc-holding unit 200 also includes drive shaft 225 mechanically coupled to motor 230. Motor 230 rotates drive shaft 225, and ultimately disc chuck 210 and disc 250 about disc chuck axis of rotation 205, as shown by arrow A. Generally, the disc may be rotated counterclockwise or clockwise about the disc chuck axis of rotation.

In some embodiments, edge-modifying unit 300 includes abrasive article support 310 mechanically coupled to drive shaft 315, which is mechanically coupled to motor 330. Motor 330 rotates drive shaft 315 and ultimately abrasive article support 310 about support axis of rotation 305, as shown by arrow B. Generally, the abrasive article support may be rotated counterclockwise or clockwise about the support axis of rotation.

In some embodiments, the abrasive article support and the disc are co-rotating, i.e., both are rotated clockwise or counterclockwise about their respective axis of rotation. In some embodiments, the abrasive article support and the disc are counter-rotating.

In some embodiments, disc chuck axis of rotation 205 and support axis of rotation 305 are co-planar. In some embodiments, disc chuck axis of rotation 205 and support axis of rotation 305 are parallel. As shown in FIG. 1, in some embodiments, co-planar disc chuck axis of rotation 205 and support axis of rotation 305 intersect forming angle X. Generally, the angle between the axes of rotation can vary from 0 degrees (i.e., parallel) to 89 degrees. In some embodiments, the angle is between about 5 degrees and about 75 degrees. In some embodiments, the angle is at least about 5 degrees, in some embodiments, at least about 30 degrees, or even at least about 40 degrees. In some embodiments, the angle is no greater than about 75 degrees, in some embodiments, no greater than about 60 degrees, or even no greater than about 50 degrees.

In some embodiments, the disc chuck axis of rotation and the support axis of rotation are not co-planar. For example, in some embodiments, the disc chuck axis of rotation also is tilted with respect to the plane initially formed by the disc chuck axis of rotation and the support axis of rotation by a further rotation about an axis, perpendicular to the disc chuck axis of rotation, which passes through the centroid of the area of contact between the disc and the abrasive article. Generally, the angle of tilt may range from about 15 degrees to about 50 degrees. In some embodiments, the tilt angle is at least about 30 degrees, and in some embodiments at least about 40 degrees.

In some embodiments, abrasive article 320 circumferentially wraps cylindrical surface 311 of abrasive article support 310. In some embodiments, the fixed abrasive article is attached to abrasive article support 310, e.g., mechanically or adhesively attached. In some embodiments, the abrasive article is a belt that only partially wraps the cylindrical surface of the support. The web path of such a belt may pass over one or more additional driven and/or idler rollers. In some embodiments, abrasive article support 310 is an idler roller. In such embodiments, the abrasive belt will cause the support to rotate as it passes over at least a portion of the cylindrical surface of the support. In some embodiments, abrasive article support 310 is a driven roller. In some embodiments, the abrasive article support will be stationary, e.g., a shoe. In some embodiments, the stationary support may include a mean to reduce friction between the abrasive article and the surface of the support as the abrasive article is passes over the support. Exemplary means to reduce friction include a fluid bearing, e.g., an air bearing.

Generally, any known abrasive article may be used. In some embodiments, the abrasive article is a fixed abrasive article. In some embodiments, the abrasive article may be a three-dimensional fixed abrasive article having abrasive particles dispersed throughout at least a portion of its thickness such that erosion exposes additional abrasive particles. The abrasive article can also be textured such that it includes raised portions and recessed portions in which at least the raised portions include abrasive particles in a binder. Exemplary fixed abrasive articles are described, e.g., in U.S. Pat. Nos. 5,014,468; 5,453,312; 5,454,844; 5,692,950; 5,820,450; and 5,958,794; and in WO 98/49723.

In some embodiments, the fixed abrasive article may include a backing. Any known backing may be used. For example, polymeric films, fabrics, metal foils, nonwovens, and combinations thereof may be used. In addition, Bruxvoort et al. in U.S. Pat. No. 5,958,764 (column 17, line 12 through column 18, line 15 of which is incorporated herein by reference) describe useful backings. Particular selection is within the skill in the art.

In some embodiments, fixed abrasive articles include abrasive composites. Abrasive composites are known in the art of fixed abrasive articles and may comprise abrasive particles dispersed throughout a binder. In some embodiments, an abrasive composite may comprise a polymeric material having separate phases, with one phase acting as abrasive particles.

Generally, any known binder may be used. For example, (meth)acrylates, epoxies, urethanes, polystyrenes, vinyls, and combinations thereof may be used. In addition, Bruxvoort et al. in U.S. Pat. No. 5,958,764 (column 22, line 64 through column 34, line 5 of which is incorporated herein by reference) describe useful binders. Particular selection is within the skill in the art.

Generally, any known abrasive particles may be used including, e.g., diamonds, silicon carbide, and/or boron nitride particles. In addition, Bruxvoort et al. in U.S. Pat. No. 5,958,764 (column 18, line 16 through column 21, line 25 of which is incorporated herein by reference) describe useful abrasive particles. In some embodiments, the abrasive particles may be in the form of abrasive agglomerates, which comprise a plurality of individual abrasive particles bonded together to form a unitary particulate mass. Examples of abrasive agglomerates are further described in U.S. Pat. Nos. 4,652,275; 4,799,939; and 5,500,273.

In some embodiments, the abrasive article comprises a compressible composite layer. In some embodiments, the composite layer includes a binder and abrasive agglomerates. In some embodiments, the composite layer comprises at least 50% by volume of the binder. In some embodiments, the binder comprises a material selected from the group consisting of urethanes, epoxies, and combinations thereof. In some embodiments, the composite layer further comprises inorganic fillers.

Exemplary abrasive agglomerates useful in a compressible composite layer include those described above. In some embodiments, the abrasive agglomerates comprise abrasive particles dispersed in an inorganic matrix. In some embodiments, the inorganic matrix is selected from the group consisting of a glass, a ceramic, and a glass ceramic. In some embodiments, the abrasive particles are diamonds, silicon carbide, and/or boron nitride particles.

In some embodiments, the abrasive articles include a support to which the compressible composite layer is bonded. In some embodiments, the compressible composite layer is directly bonded to the support. In some embodiments, one or more bonding layers are interposed between the compressible composite layer and the support.

Various embodiments of the edge modifying apparatus of present disclosure provide a relative motion between the edge of the disc being modified and the modifying surface, e.g., the abrasive surface of a fixed abrasive article via the co-rotation or counter rotation of the disc and the fixed abrasive article support. In some embodiments, it may be desirable to provide additional relative motion between the edge of the disc being modified and the modifying surface, e.g., the abrasive surface of a fixed abrasive article. For example, in some embodiments, it may be desirable to linearly oscillate the disc and the fixed abrasive article relative to one another.

In some embodiments, this linear oscillation may provide additional disc edge modification due to the motion of the edge of the disc relative to the abrasive surface of the fixed abrasive article. In some embodiments, the linear oscillation may enable a greater area of the fixed abrasive article to contribute to the edge modifying process.

In some embodiments, the linear oscillation may reduce or eliminate grooving of the fixed abrasive article. “Grooving” arises when the edge of a disc is pressed against the abrasive surface of fixed abrasive article and a relative rotational motion is created between the edge of the disc and the abrasive surface. As the edge of the disc is modified, the abrasive article may develop a wear pattern corresponding to the area of contact between the edge of the disc and the fixed abrasive article. If the edge of the disc is fixed relative to the surface of the fixed abrasive article, this can lead to a groove in the surface of the fixed abrasive article. Such a groove may detrimentally affect the edge polishing operation and can require premature replacement of the abrasive article.

Generally, linear oscillation of the edge of a disc relative to the surface of the abrasive article effectively expands the area of contact between the edge of the disc and the abrasive article. In some embodiments, this leads to the more uniform wearing of the surface of the abrasive article, which may reduce or eliminate grooving.

In some embodiments, the disc may be oscillated relative to an abrasive article held in a fixed position. In some embodiments, the abrasive article may be oscillated relative to a disc held in a fixed position. In some embodiments, both the abrasive article and the disc may be oscillated relative to each other.

One means to linearly oscillate the edge of a disc relative to the surface of the abrasive article is shown in FIG. 1. Referring to FIG. 1, motor 330 is shown mounted to frame 333 on tracks 335. Motor 330 is mechanically coupled to cam follower 340, which is mechanically coupled to peripheral edge 346 of cam 345. Cam shaft 347 mechanically couples cam 345 to motor 349. As motor 349 rotates cam 345 via cam shaft 347, cam follower 340 tracks peripheral edge 346 of cam 345. Cam follower 340 translates the rotational motion of cam 345 into a linear motion, which ultimately results in the oscillation of abrasive article support 310 and abrasive article 320 relative to disc 250 (as shown by arrow C). In some embodiments, e.g., an eccentric drive, the cam follower may be attached to the cam at some location between the center and the peripheral edge of the cam rather than tracking the cam's edge.

In some embodiments, it may be desirable to have a substantially constant linear oscillation velocity across the face of the abrasive article during each half-cycle of oscillation. Generally, a substantially constant linear oscillation velocity will result in more evenly worn abrasive article, and may create a more uniform polish of the workpiece.

In some embodiments, it may be desirable to oscillate the disc relative to the abrasive article and/or the abrasive article relative to the disc using a uniform velocity oscillation driver coupled to at least one of the disc chuck and the abrasive article support. As used herein, “a uniform velocity linear oscillation driver” provides a substantially constant oscillation velocity during at least 80% of each half-cycle of oscillation. As used herein, the linear oscillation velocity is “substantially constant” if it varies less than ±5% from the average linear oscillation velocity over the period of interest. In some embodiments, the linear oscillation velocity varies by less than ±3%, or even by less than ±1% from the average linear oscillation velocity over the period of interest

In some embodiments, the uniform velocity oscillation driver may comprise one or more of a stepper motor, a screw drive, a magnetic drive, a linear gear, and the like. Generally, selection of the means used to oscillate is within the ability of one of ordinary skill in the art.

In some embodiments, the uniform oscillation driver comprises a reversible motor. The motor may be either a direct current (DC) or alternating current (AC) motor. In some embodiments, the motor is a stepping motor. In some embodiments, the uniform oscillation driver comprises controls to reverse the direction of the reversible motor. For example, in some embodiments, the controls may comprise limit switches. In some embodiments, the controls may comprise a programmed stepping motor controller. In some embodiments, the uniform velocity oscillation driver comprises an eccentric driven by a stepping motor. In some embodiments, the stepper motor is driven with a pulse chain that compensates for the non-linear movement of the eccentric such that the oscillation speed is linear between the reversals.

In some embodiments, the uniform velocity oscillation driver may comprise a cam coupled to a cam follower. A cam can be created to deliver a substantially constant linear oscillation velocity across the abrasive article. To design such a cam one would start with the traverse distance and the minimum radius of the cam compatible with the mechanical components of the cam drive and the cam follower. For example if a cam is to have a 2 unit throw (wherein a “unit” is an arbitrary measure of distance), a single lobe (one traverse per cam rotation), and a minimum radius of 1 unit; then the radius needs to increase linearly as a function of angle from 1 unit to 3 units for 180 degrees of the cam and then decrease linearly back to 1 unit over the angle 180 degrees to 360 degrees. Table 1 shows the desired cam radius as a function of the angle for such a cam.

TABLE 1 Radius as a function of angle for a cam generating a substantially constant oscillation velocity. Angle Radius X axis Y axis 0 1.000 1.000 0.000 30 1.333 1.155 0.667 60 1.667 0.833 1.443 90 2.000 0.000 2.000 120 2.333 −1.167 2.021 150 2.667 −2.309 1.333 180 3.000 −3.000 0.000 210 2.667 −2.309 −1.333 240 2.333 −1.167 −2.021 270 2.000 0.000 −2.000 300 1.667 0.833 −1.443 330 1.333 1.155 −0.667 360 1.000 1.000 0.000

As shown in Table 1, the polar coordinates can be converted to Cartesian coordinates. A cam can then be made by entering the numbers from such a table into the spline function of a drawing program, printing out a full size drawing, adhering the drawing to a cam substrate (e.g., a piece of sheet metal), and cutting out the cam with, e.g., a band saw, by following the line of the drawing. Other means of creating cam of a desired shape are known in the relevant art.

The movement of a cam follower tracking the peripheral edge of the cam described by Table 1 is shown in FIG. 2. As shown, displacement 400 increases linearly during first half-cycle of oscillation 410 (i.e., as the cam rotates from 0 degrees to 180 degrees) and decreases linearly during second half cycle of oscillation 420 (i.e., as the cam rotates from 180 degrees to 360 degrees).

Because the displacement is linear, the rate of change in displacement per unit time is substantially constant during each half-cycle of rotation. As a practical matter, it may be difficult or undesirable to make the sharp transition in velocity between half-cycles of oscillation (see, e.g., FIG. 2, point 430). In some embodiments, the shape of the cam may be rounded at these locations to allow a more gradual transition.

For example, a cam was created to deliver a substantially constant oscillation velocity across the abrasive article. To design such a cam one would start with the traverse distance and the minimum radius of the cam compatible with the mechanical components of the cam drive and the cam follower. The desired radius of a function of angle is then generated, and the polar coordinates are converted to x-y coordinates, as shown in Table 2 for a cam having a minimum radius of 6.4 millimeters (mm) (0.25 inches), and a maximum radius of 57.2 mm (2.25 inches).

Using the x-y coordinates provided in Table 2, AutoCad LT generated the shape of a cam using a spline function. The spline fit automatically rounded the cam shape at 0 and 180 degrees. A cam was made by the manner described above and had the heart shape shown in FIG. 3. Referring to FIG. 3, cam 600 includes peripheral edge 620. Cam 600 is mounted to a shaft via mounting hole 610. Distance 605 between rotational centerpoint 601 and peripheral edge 620 increases substantially linearly from minimum distance 602 to maximum distance 603 during the first half-cycle of oscillation. During the second half-cycle of oscillation, distance 605 decreases substantially linearly from maximum distance 603 to minimum distance 602. Generally, a cam made with a larger minimum radius would provide a more uniform velocity when a finite size cam follower is used.

Peripheral edge 620 includes rounded portions 621 and 622 at the locations of minimum distance and maximum distance, respectively. Depending, e.g., on the degree of rounding of the peripheral edge of the cam at the transition regions between the first and second half-cycles of oscillation, the linear oscillation velocity will not be constant in these transition regions. In some embodiments, the peripheral edge is defined by a curve resulting in a substantially constant linear oscillation velocity during at least 80% of each half-cycle of oscillation between the transition regions. In some embodiments, the peripheral edge is defined by a curve resulting in a substantially constant linear oscillation velocity during at least 90% of each half-cycle of oscillation. In some embodiments, the peripheral edge is defined by a curve resulting in a substantially constant linear oscillation velocity during at least 95%, or even at least 98%, of each half-cycle of oscillation.

TABLE 2 Radius as a function of angle for a cam generating a substantially constant oscillation velocity. millimeters angle radius x y 0 6.4 6.4 0.0 10 9.2 9.0 1.6 20 12.0 11.3 4.1 30 14.8 12.8 7.4 40 17.6 13.5 11.3 50 20.5 13.2 15.7 60 23.3 11.6 20.2 70 26.1 8.9 24.5 80 28.9 5.0 28.5 90 31.8 0.0 31.8 100 34.6 −6.0 34.0 110 37.4 −12.8 35.1 120 40.2 −20.1 34.8 130 43.0 −27.7 33.0 140 45.9 −35.1 29.5 150 48.7 −42.2 24.3 160 51.5 −48.4 17.6 170 54.3 −53.5 9.4 180 57.2 −57.2 0.0 190 54.3 −53.5 −9.4 200 51.5 −48.4 −17.6 210 48.7 −42.2 −24.3 220 45.9 −35.1 −29.5 230 43.0 −27.7 −33.0 240 40.2 −20.1 −34.8 250 37.4 −12.8 −35.1 260 34.6 −6.0 −34.0 270 31.8 0.0 −31.8 280 28.9 5.0 −28.5 290 26.1 8.9 −24.5 300 23.3 11.6 −20.2 310 20.5 13.2 −15.7 320 17.6 13.5 −11.3 330 14.8 12.8 −7.4 340 12.0 11.3 −4.1 350 9.2 9.0 −1.6 360 6.4 6.4 0.0

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. 

1. An edge modifying apparatus comprising: (a) a disc chuck having a disc-mounting surface and a disc chuck axis of rotation; (b) a disc edge modifying unit comprising an abrasive article support having a support surface and a support axis of rotation; and (c) a uniform velocity linear oscillation driver coupled to at least one of the disc chuck and the abrasive article support.
 2. The apparatus of claim 1, wherein the uniform velocity linear oscillation driver is mechanically coupled to the abrasive article support.
 3. The apparatus of claim 1, wherein the uniform velocity linear oscillation driver comprises a cam having a peripheral edge defined by a curve resulting in a substantially constant linear oscillation velocity during at least 80% of each half-cycle of oscillation.
 4. The apparatus of claim 3, further comprising a cam follower mechanically coupled to the peripheral edge of the cam, wherein the peripheral edge is defined by a curve resulting in a substantially constant linear oscillation velocity during at least 90% of each half-cycle of oscillation.
 5. The apparatus of claim 1, wherein the disc chuck axis of rotation forms an angle of between about 30 degrees and about 75 degrees, inclusive, relative to the support axis of rotation.
 6. The apparatus of claim 1, wherein the disc chuck axis of rotation and the support axis of rotation are coplanar.
 7. The apparatus of claim 1, wherein the disc chuck axis of rotation is tilted by between about 15 degrees and about 50 degrees, inclusive.
 8. The apparatus of claim 1 further comprising a fixed abrasive article adjacent at least a portion of the support surface.
 9. The apparatus of claim 1, wherein the uniform velocity linear oscillation driver comprises at least one of a stepper motor, a screw drive, a magnetic drive, and a linear gear.
 10. The apparatus of claim 1, wherein the uniform velocity linear oscillation driver comprises a reversible motor and a control to reverse the direction of the reversible motor.
 11. (canceled)
 12. (canceled)
 13. (canceled)
 14. A method of modifying the edge of a disc comprising: (a) providing an edge modifying apparatus comprising: (i) a disc chuck including a disc-mounting surface and a disc chuck axis of rotation; (ii) a disc edge modifying unit comprising an abrasive article support having a support surface and a support axis of rotation; and (iii) a uniform velocity linear oscillation driver coupled to at least one of the disc chuck and the abrasive article support; (b) centering a disc on the disc-mounting surface of the disc chuck; (c) positioning an edge of the disc relative to a fixed abrasive article adjacent the support surface of the abrasive article support such that at least a portion of the edge of the disc contacts an abrasive surface of the fixed abrasive article; (d) rotating the disc chuck; and (e) providing a linear oscillation of the fixed abrasive support surface relative to the disc-mounting surface, wherein the linear oscillation has a substantially constant velocity for at least 80% of each half-cycle of oscillation.
 15. The method of claim 14, wherein creating a linear oscillation of the fixed abrasive support surface relative to the disc-mounting surface comprises oscillating the fixed abrasive article support.
 16. The method of claim 14 further comprising rotating the abrasive article support.
 17. The method of claim 14, wherein the uniform velocity linear oscillation driver comprises a cam having a peripheral edge.
 18. The method of claim 17, wherein the uniform velocity linear oscillation driver further comprising a cam follower mechanically coupled to the peripheral edge of the cam, and wherein the peripheral edge is defined by a curve resulting in a substantially constant oscillation velocity during at least 90% of each half-cycle of oscillation.
 19. The method of claim 14, wherein the uniform velocity linear oscillation driver comprises at least one of a stepper motor, a screw drive, a magnetic drive, and a linear gear.
 20. The method of claim 14, wherein the uniform velocity linear oscillation driver comprises a reversible motor and a control to reverse the direction of the reversible motor.
 21. The apparatus of claim 1, further comprising a positioning assembly coupled to the disc chuck. 